1. Field of the Invention
This invention relates to the field of switching power converters, and particularly to circuits for generating a reference that can be used to limit the converter's input or output current.
2. Description of the Related Art
Switching power converters often are equipped with one or more current limiting circuits. For example, a switching power converter's input and/or output current may be limited to prevent damage to one or more circuit components, or for the purpose of minimizing power consumption.
One way in which current limiting is provided in switching power converters is with the use of a peak current limiting scheme. For example, for a DC-DC boost converter having a current-mode architecture, the inductor current is compared against a fixed reference, and if the current exceeds the reference, the power switch on-time is terminated. This method allows inductor current to be regulated to the peak current limit; the comparator output can also be used to indicate that the peak current limit has been exceeded.
A similar technique can be implemented for valley-current converters. This method of current detection and limiting addresses peak or valley current in the inductor, rather than DC current. This means that the effective DC current load on the system while operating in a current-limiting mode will vary with inductor value, switching frequency and the converter's input and output voltage. In addition, due to the ripple current in the inductor, peak or valley limiting schemes such as those described above can result in the effective DC (or average) current limit varying with the input and output voltages of the converter, as well as with variations in inductor value and changes in the duty cycle.
Another technique which may be used for current limiting in a DC-DC boost converter is to use information obtained from the output of the error amplifier in the control loop. This signal contains information about the peak inductor current (or valley current) in a current-mode converter. This method of current detection and limiting suffers from the same limitations as the peak (or valley) current limiting method described above.